25. What is the difference between 'let', 'const', and 'var' in JavaScript?

In JavaScript, the keywords let, const, and var are used to declare variables, but they have different behaviors in terms of scope, hoisting, and reassignability.

var

• Scope: var is function-scoped, which means it is accessible within the function it was declared, or globally if not inside a function.
• Hoisting: Variables declared with var are hoisted to the top of their current scope. This means the declaration is moved to the top, but not the initialization.
• Reassignability: Variables declared with var can be re-assigned and updated.

Example:

function exampleVar() {
  console.log(x); // undefined
  var x = 10;
  console.log(x); // 10
}
exampleVar();

let

• Scope: let is block-scoped, meaning it is only accessible within the block, statement, or expression where it is used.
• Hoisting: let is hoisted, but unlike var, it is not initialized. Accessing it before declaration results in a ReferenceError.
• Reassignability: Variables declared with let can be re-assigned.

Example:

function exampleLet() {
  if (true) {
    let y = 20;
    console.log(y); // 20
  }
  console.log(y); // ReferenceError: y is not defined
}
exampleLet();

const

• Scope: Similar to let, const is block-scoped.
• Hoisting: const is also hoisted but not initialized, similar to let.
• Reassignability: Variables declared with const cannot be re-assigned. However, if the variable is an object or an array, the properties or elements can be modified.

Example:

function exampleConst() {
  const z = 30;
  console.log(z); // 30
  // z = 40; // TypeError: Assignment to constant variable.

  const arr = [1, 2, 3];
  arr.push(4);
  console.log(arr); // [1, 2, 3, 4]
}
exampleConst();

Summary

• Use var if you need function-scoped variables (rarely recommended).
• Use let for variables that will change over time and are block-scoped.
• Use const for variables that should not be reassigned, ensuring immutability for primitive values.